Which of the following is the MOST likely cause of users being unable to verify a single user's email signature and that user being unable to decrypt sent messages?

In asymmetric cryptography, a sender encrypts a message with the recipient's public key and the recipient decrypts with their matching private key - if the keys do not mathematically correspond, decryption fails. Similarly, a digital signature is created with a private key and verified with the paired public key - a mismatch means verification fails for all recipients. Both symptoms described in the question - failed signature verification by others and failed decryption by the user - point precisely to a public/private key pair mismatch.

Key escrow stores backup copies of keys for authorized recovery; corruption would affect recovery scenarios but would not cause both signature verification failure and decryption failure simultaneously.

A weak public key reduces cryptographic strength but does not cause complete decryption failure or prevent signature verification.

A weak private key reduces security against brute-force attacks but still functions for signing and decryption as long as the key pair is matched.